This invention relates to a solid-state image pickup apparatus, a driving method for a solid-state image pickup apparatus and an electronic device.
A solid-state image pickup apparatus such as a CMOS (Complementary Metal Oxide Semiconductor) image sensor which is a kind of solid-state image pickup apparatus of the X-Y address type carries out operation of sequentially scanning photocharge generated by and accumulated in a photoelectric conversion portion for each pixel or for each row. In this sequential scanning, where a rolling shutter is used as an electronic shutter, the start time and the end time of exposure for accumulating photocharge cannot be made coincide among all pixels. Therefore, the sequential scanning has a problem in that, upon image pickup of an image pickup object which is moving, a picked up image suffers from distortion.
For image pickup of an image pickup object moving at a high speed which cannot permit image distortion of the type described or for sensing applications which require simultaneity of a picked up image, a global shutter which executes starting of exposure and ending of exposure at the same timings with regard to all pixels in a pixel array is adopted as an electronic shutter. In order to implement the global shutter an embedded MOS (Metal Oxide Semiconductor) capacitor is provided as a region for accumulating photocharge, that is, as a charge accumulation portion, separately from a photodiode which is a photoelectric conversion portion, as disclosed in Japanese Patent No. 3874135 (hereinafter referred to as Patent Document 1).
However, in order for the embedded MOS capacitor to accept, upon global shutter operation, all photocharge generated by photoelectric conversion by and accumulated in the photodiode, the embedded MOS capacitor has to have a saturation charge amount greater than that of the photodiode. Conversely speaking, where the unit pixel size is same, the area of the photodiode is reduced significantly due to the presence of the embedded MOS capacitor in the unit pixel. Therefore, there is a problem that the saturation charge amount of the photodiode is reduced.
As a countermeasure, a technique that photocharge generated by photoelectric conversion in the photodiode is accumulated in both of the photodiode and the embedded MOS capacitor has been proposed and is disclosed in Japanese Patent Laid-Open No. 2009-268083 (hereinafter referred to as Patent Document 2). According to this related art, the saturation charge amount is equal to the sum of the saturation charge amount of the photodiode and the saturation charge amount of the embedded MOS capacitor.
However, the related art disclosed in Patent Document 2 is significantly inferior in terms of the saturation charge amount if it is compared with a CMOS image sensor which does not have the global shutter function. This is because, in order to achieve the global exposure, it is necessary to additionally provide not only a charge accumulation portion, which is, in the related art, the embedded MOS capacitor, but also a transistor in the unit pixel.
Separately from the related art which achieves global exposure described above, also it can be imagined readily to use not an embedded MOS capacitor but a capacitor having a higher capacitance value per unit area as the charge accumulation portion. However, a capacitor having a higher capacitance value per unit area generally exhibits high leak current, and this makes a problem that deterioration of properties at dark such as dark current or white spots becomes conspicuous.
Therefore, it is desirable to provide a solid-state image pickup apparatus which can assure an increased saturation charge amount without deterioration of the picture quality of a picked up image at dark or in a low illuminance state, a driving method for the solid-state image pickup apparatus and an electronic device which includes the solid-state image pickup apparatus.